With increasing needs for the reduction of an automobile weight, a piece of steel having a higher strength and less weight for a given size is increasingly desired. Strengthening of a steel sheet makes it possible to reduce an automobile's weight through reducing the thickness of the steel sheet material and increase the automobile's collision safety. In this regard, attempts have been made recently to form components of complicated shapes by applying a hydroforming method to high strength steel pipes. These processes aim to reduce the number of components, the number of welded flanges and the like in order to conform with the increasing needs for automobile weight reduction and cost reduction.
Actual application of such new forming technologies as the hydro forming method is expected to bring about significant advantages such as the reduction of cost and the expansion of design freedom. In order to fully take advantage of the hydroforming method, new materials suitable for use in this new hydroforming method are desired.
However, if it is attempted to obtain a steel sheet having a high strength and being excellent in formability, particularly deep draw ability, it has been essentially required to use an ultra-low-carbon steel containing a very small amount of C and to strengthen it by adding elements such as Si, Mn and P, as disclosed in Japanese Unexamined Patent Publication No. S56-139654, for example.
Reducing the amount of C used in the steel requires the use of vacuum degassing in the steelmaking process. During the vacuum degassing process, CO2 gas is emitted in quantity. Emitting the CO2 gas is not environmentally friendly and may have substantial negative effects as to the conservation of the global environment.
Meanwhile, steel sheets that have comparatively high amounts of C and yet exhibit good deep drawability have been disclosed. Such steel sheets have been disclosed in Japanese Examined patent Publication Nos. S57-47746, H2-20695, S58-49623, S61-12983 and H1-37456, Japanese Unexamined patent Publication No. S59-13030 and others. However, even in these comparatively high C steel sheets, the amounts of C are 0.07% or less, making these comparatively high C steel sheets very-low-carbon steel sheets. Further, Japanese Examined Patent Publication No. S61-10012 discloses that a comparatively good r-value is obtained even with a C amount of 0.14%. However, the disclosed steel contains P in quantity, thereby causing the deterioration of secondary workability, problems with weldability and fatigue strength after welding in some cases. The present inventors have applied a technology to solve these problems in Japanese Patent Application No. 2000-403447.
Further, the present inventors have filed another patent application, Japanese Patent Application No. 2000-52574, regarding a steel pipe that has a controlled texture and excellent formability. However, such a steel pipe finished through high-temperature processing often contains solute C and solute N in quantity. These solute elements sometimes cause cracks to be generated during hydroforming and surface defects such as stretcher strain may be induced. Other problems with such a steel pipe include deteriorated productivity due to high-temperature thermo-mechanical treatment applied after a steel sheet has been formed into a tubular shape, negative effects on the global environment, increased cost, and the like.